Effect of bumetanide on the positive endocochlear dc potential of the cat

Evaluation of bumetanide as a potential therapeutic agent for Alzheimer's disease

Therapeutics discovery and development for Alzheimer's disease (AD) has been an area of intense research to alleviate memory loss and the underlying pathogenic processes. Recent drug discovery approaches have utilized in silico computational strategies for drug candidate selection which has opened the door to repurposing drugs for AD. Computational analysis of gene expression signatures of patients stratified by the APOE4 risk allele of AD led to the discovery of the FDA-approved drug bumetanide as a top candidate agent that reverses APOE4 transcriptomic brain signatures and improves memory deficits in APOE4 animal models of AD. Bumetanide is a loop diuretic which inhibits the kidney Na+-K+-2Cl- cotransporter isoform, NKCC2, for the treatment of hypertension and edema in cardiovascular, liver, and renal disease. Electronic health record data revealed that patients exposed to bumetanide have lower incidences of AD by 35%-70%. In the brain, bumetanide has been proposed to antagonize the NKCC1 isoform which mediates cellular uptake of chloride ions. Blocking neuronal NKCC1 leads to a decrease in intracellular chloride and thus promotes GABAergic receptor mediated hyperpolarization, which may ameliorate disease conditions associated with GABAergic-mediated depolarization. NKCC1 is expressed in neurons and in all brain cells including glia (oligodendrocytes, microglia, and astrocytes) and the vasculature. In consideration of bumetanide as a repurposed drug for AD, this review evaluates its pharmaceutical properties with respect to its estimated brain levels across doses that can improve neurologic disease deficits of animal models to distinguish between NKCC1 and non-NKCC1 mechanisms. The available data indicate that bumetanide efficacy may occur at brain drug levels that are below those required for inhibition of the NKCC1 transporter which implicates non-NKCC1 brain mechansims for improvement of brain dysfunctions and memory deficits. Alternatively, peripheral bumetanide mechanisms may involve cells outside the central nervous system (e.g., in epithelia and the immune system). Clinical bumetanide doses for improved neurological deficits are reviewed. Regardless of mechanism, the efficacy of bumetanide to improve memory deficits in the APOE4 model of AD and its potential to reduce the incidence of AD provide support for clinical investigation of bumetanide as a repurposed AD therapeutic agent.

Evaluation and Management of Tinnitus: Are There Opportunities for Improvement?

OBJECTIVE

To review current information about diagnosis and management of tinnitus aiming to identify opportunities for achieving a cost-effective, efficient, evidence-based approach that meets the needs of tinnitus sufferers.

DATA SOURCES

PubMed/MEDLINE.

REVIEW METHODS

In total, 249 relevant published reports were reviewed. Pertinent keywords and MeSH terms identified reports via PubMed and EMBASE. Acknowledged experts were consulted on ways to improve tinnitus management.

CONCLUSIONS

There may be opportunities to improve evaluation and management of patients with tinnitus using modern modes of communication and a multidisciplinary therapeutic approach.

IMPLICATIONS FOR PRACTICE

Tinnitus can adversely affect quality of life while being time-consuming and costly to evaluate and manage. Based on both personal experience and the reports of others, patients with tinnitus who choose to see a physician primarily want to know two things: (1) that the tinnitus that is so distressing will not remain at the same level of severity forever and (2) that something can be done to help cope with the tinnitus that is so annoying. Recent advancements in internet communications, social media, information technology, artificial intelligence, machine learning, holistic medical care, mind-body integrative health care, and multidisciplinary approaches in medical therapeutics may be possibly making new ways of meeting the needs of patients with tinnitus.

Computational model of vectorial potassium transport by cochlear marginal cells and vestibular dark cells

Cochlear marginal cells and vestibular dark cells transport potassium into the inner ear endolymph, a potassium-rich fluid, the homeostasis of which is essential for hearing and balance. We have formulated an integrated mathematical model of ion transport across these epithelia that incorporates the biophysical properties of the major ion transporters and channels located in the apical and basolateral membranes of the constituent cells. The model is constructed for both open- and short-circuit situations to test the extremes of functional capacity of the epithelium and predicts the steady-state voltages, ion concentrations, and transepithelial currents as a function of various transporter and channel densities. We validate the model by establishing that the cells are capable of vectorial ion transport consistent with several experimental measurements. The model indicates that cochlear marginal cells do not make a significant direct contribution to the endocochlear potential and illustrates how changes to the activity of specific transport proteins lead to reduced K(+) flux across the marginal and dark cell layers. In particular, we investigate the mechanisms of loop diuretic ototoxicity and diseases with hearing loss in which K(+) and Cl(-) transport are compromised, such as Jervell and Lange-Nielsen syndrome and Bartter syndrome, type IV, respectively. Such simulations demonstrate the utility of compartmental modeling in investigating the role of ion homeostasis in inner ear physiology and pathology.

Mice lacking the basolateral Na-K-2Cl cotransporter have impaired epithelial chloride secretion and are profoundly deaf

In chloride-secretory epithelia, the basolateral Na-K-2Cl cotransporter (NKCC1) is thought to play a major role in transepithelial Cl(-) and fluid transport. Similarly, in marginal cells of the inner ear, NKCC1 has been proposed as a component of the entry pathway for K(+) that is secreted into the endolymph, thus playing a critical role in hearing. To test these hypotheses, we generated and analyzed an NKCC1-deficient mouse. Homozygous mutant (Nkcc1(-/-)) mice exhibited growth retardation, a 28% incidence of death around the time of weaning, and mild difficulties in maintaining their balance. Mean arterial blood pressure was significantly reduced in both heterozygous and homozygous mutants, indicating an important function for NKCC1 in the maintenance of blood pressure. cAMP-induced short circuit currents, which are dependent on the CFTR Cl(-) channel, were reduced in jejunum, cecum, and trachea of Nkcc1(-/-) mice, indicating that NKCC1 contributes to cAMP-induced Cl(-) secretion. In contrast, secretion of gastric acid in adult Nkcc1(-/-) stomachs and enterotoxin-stimulated fluid secretion in the intestine of suckling Nkcc1(-/-) mice were normal. Finally, homozygous mutants were deaf, and histological analysis of the inner ear revealed a collapse of the membranous labyrinth, consistent with a critical role for NKCC1 in transepithelial K(+) movements involved in generation of the K(+)-rich endolymph and the endocochlear potential.

Molecular and clinical implications of loop diuretic ototoxicity

Recent advances in molecular biology have been applied to inner ear research. Loop diuretic ototoxicity has been suggested, but not proven, to share a common mechanism with diuretic effects on renal tubules. The discovery of the molecular nature of the Na-K-2Cl cotransporter in the cochlea provided a better understanding of loop diuretic ototoxicity. In this review, we describe clinical reports of loop diuretic ototoxicity and other information obtained by physiological, biochemical and morphological investigations related to the mechanism sensitive to loop diuretics. Based on recent evidence for the molecular nature of the Na-K-2Cl cotransporter expressed in the mammalian cochlea, the underlying mechanisms of ototoxicity induced by loop diuretics are described.

Ototoxic effect of potassium canrenoate on the guinea pig cochlea

Potassium canrenoate (PC) is a diuretic with antialdosterone action, reducing the reabsorption of sodium in the efferent renal tubules. This drug has been reported to damage only the marginal cells of the stria vascularis in the cochlea. The perilymphatic space of the guinea pig cochlea was perfused with an artificial perilymph containing 5 x 10(-3) M potassium canrenoate. Following the onset of perfusion, the endocochlear dc potential (EP) gradually declined to around 10 mV but did not become negative even when the perfusion was continued. A similar decrease in EP was observed in kanamycin-deafened guinea pigs. When the respirator was turned off and perfusion discontinued, a large negative EP appeared during 5 min of anoxia in normal guinea pigs but not in the kanamycin-deafened guinea pigs. The decreased EP recovered to preanoxic level after resumption of ventilation. The cochlear microphonics (CM) also gradually declined in parallel with the EP. The summating potential (SP) showed only a minor change. The potassium ion activity in the endolymph decreased slightly but the sodium ion activity remained unchanged during perfusion. These findings suggest that the main target of the PC is the cells of the stria vascularis.

Comparative ototoxicity of furosemide and piretanide

The purpose of these studies was to compare the ototoxicity of piretanide to that of furosemide in the chinchilla. Chinchillas weighing 400 to 700 g were anesthetized with ketamine plus pentobarbital. Endocochlear potential (EP) was measured continuously by the round window approach using glass microelectrodes. Piretanide or furosemide injected through a jugular vein catheter in adults range from 10-100 mg/kg. Little or no change in EP was noted with doses below 15 mg/kg of either diuretic. The overall pattern of decline of EP was rather similar in piretanide-treated and furosemide-injected animals, and it was interesting to find that the dose-ototoxicity response curve for piretanide was similar to that for furosemide. The diuretic effect of equal ototoxic doses of either diuretic was equivalent. Both diuretics cause a decrease of endocochlear potential when applied locally to the round window membrane of the chinchilla as well as by systemic administration. These findings suggest that piretanide and furosemide have approximately the same propensity for ototoxicity.

Quantitative evaluation of ototoxic side effects of furosemide, piretanide, bumetanide, azosemide and ozolinone in the cat--a new approach to the problem of ototoxicity

A new method for the quantitative assessment of acute ototoxic side effects of drugs is described. It is suitable for screening purposes. The method is based on the determination of the toxic dose (TD50) which causes a defined hearing loss in 50% of the animals tested. The hearing loss is defined as a complete suppression of the compound action potential (CAP) of the auditory nerve, elicited by clicks 30 dB above threshold. This is approximately equivalent to a clinical hearing loss of 30 dB. The TD50 is used to estimate the therapeutic range. With this approach ototoxic side effects of furosemide, piretanide and bumetanide were compared quantitatively in cats. The TD50 values for CAP suppression were 18.37 mg/kg for furosemide; 4.29 mg/kg for piretanide and 2.21 mg/kg for bumetanide. As equipotent diuretic doses are 2.61 mg/kg for furosemide, 0.26 mg/kg for piretanide and 1.16 mg/kg for bumetanide, it appears that the relative ototoxicity is least for piretanide and highest for bumetanide. Plasma concentrations, determined initially and when recovery of CAP to 50% of control had occurred, indicate that bumetanide may be more slowly eliminated from the cochlear spaces than furosemide and piretanide. In addition azosemide and ozolinone were tested. The TD50 for azosemide was less than 10 mg/kg. With ozolinone where there are two isomers, only the diuretic (-)ozolinone was ototoxic; the TD50 was less than 100 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

The site of involvement of hypertension within the cochlea. A comparative study of normotensive and spontaneously hypertensive rats

The function and morphology of the cochlea of the spontaneously hypertensive rat (SHR) were examined and compared with the age-paired normotensive Wistar Kyoto rat (WKY). Electro- cochleographic study revealed that the function of the cochlea in the SHR declined with increasing age to a greater extent than that of WKY. Electronmicroscopic study revealed that the primary site of the cochlear deterioration of the SHR was the vascular stria, followed by the organ of Corti. Some hypotheses to explain this phenomenon are proposed for further study.

The effect of bumetanide on the stria vascularis: a stereological analysis of cell volume density

The purpose of this research was to determine the effect of bumetanide on the volume density (Vv) of the cells, capillaries and intercellular spaces of the stria vascularis (SV). 29 chinchillas were divided into seven groups. There were 3 experimental groups, three control groups and one normal, untreated, group of animals. After either a 20 mg/kg intravenous injection of bumetanide or an injection of a control solution, the animals were killed at 10 min, 1 h and 24 h. One complete radial section of the SV was analyzed in each animal. This section was located at 70% of the length of the basilar membrane as measured from the cochlear apex. Marginal cell volume decreased by 24% and 15% at 10 min and 1 h, respectively, after bumetanide administration. Intermediate cell volume increased by 31% and 27% at 10 min and 1 h, respectively, after bumetanide administration. Intercellular space volume increased by 14% and 21% at 10 min and 1 h, respectively, after bumetanide administration. No significant alteration in the Vv was observed in the strial capillaries or basal cells. A hypothetical model of the ion transporting properties of the SV is presented.

Pharmacology, therapeutic efficacy, and adverse effects of bumetanide, a new "loop" diuretic

Bumetanide is a recently developed natriuretic and diuretic agent, belonging to the "loop" class of diuretics. Since it is rapidly and almost completely absorbed after oral administration, oral and parenteral formulations have a similar pharmacokinetic profile. Peak plasma levels are achieved approximately 30 min after oral administration. The apparent half-life is 1.2-1.5 hr, and the volume of distribution is about 25 liters. Plasma clearance is 228-255 ml/min. Bumetanide is promptly and almost completely eliminated by metabolism of the butyl side chain and urinary excretion of the parent drug and its metabolites. The principle renal site of action is the ascending limb of the loop of Henle, with a minor effect on the proximal tubule. The drug causes decreases in both free water clearance (during water diuresis) and solute free water reabsorption (during hydropenia), increased fractional delivery of sodium chloride to the distal tubule and a natriuresis approaching 20% of the filtered load of sodium, calciuria, phosphaturia, and minimal bicarbonaturia. Extensive clinical studies have been conducted with both oral and parenteral bumetanide in patients with a variety of edematous conditions. The agent has been clearly demonstrated to be an effective diuretic in the treatment of edema due to cardiac disease (congestive heart failure) and edema, with or without ascites, due to hepatic disease. Bumetanide has also been shown effective in treating edema due to renal disease, even when modest to severe renal insufficiency is present, and it may be useful in the treatment of edema refractory to other loop diuretics. As would be predicted for any potent diuretic, bumetanide administration has been associated with hypokalemia, hypochloremia, metabolic aklalosis, hyperuricemia, and prerenal azotemia. Alterations in glucose metabolism are an inconsistent finding. Transient thrombocytopenia and granulocytopenia have been noted, but no consistent or important alterations in biochemical parameters have been observed. In some patients, especially those with renal failure receiving high doses, myalgias and muscle tenderness have been described. To date only a very limited potential for ototoxicity has been observed. Bumetanide has been administered without difficulty to patients having side effects from other loop diuretics.

Ultrastructural and electrophysiological studies of acute ototoxic effects of furosemide

Previous studies of the effects of ethacrynic acid on the inner ear following intraperitoneal injection of the diuretic have shown a progression of reversible changes occurring in the stria vascularis. The time course of these changes approximately parallels alterations in endolymphatic potential (EP). In this report, some preliminary findings concerning the effects of furosemide after intraperitoneal injection of 80 mg/kg are described. EP declined over a longer time course than that recorded with intravenous injection. Cochlear microphonic (CM) and compound action potential (CAP) also declined but to differing degrees. In the stria vascularis a progression of changes was apparent. In general, the changes were similar to those observed following ethacrynic acid intoxication and affected marginal cells, intermediate cells and strial capillaries. The upper basal turn of the cochlea was affected first and the damage spread apically. In the organ of Corti, stereocilia on the outermost row of outer hair cells were disorganized. This was apparent in approximately the same region as initial strial effects and was only observed when strial derangement was quite marked.

Comparative ototoxicity of bumetanide and furosemide when used in combination with kanamycin

The ototoxicity of bumetanide and furosemide was compared in Topeka strain guinea pigs pretreated with kanamycin. The animals, anesthetized with pentobarbital, received a single dose of 400 mg/kg kanamycin subcutaneously and the diuretics via indwelling catheter in the jugular vein 2 hours later. Ototoxic drug effects were determined by measuring the electrophysiological responses of the cochlea to sound stimuli and by determining the presence or absence of cochlear sensory hair cells from the organ of Corti. Both bumetanide and furosemide produced permanent alteration of cochlear activity in the kanamycin-pretreated animals. The ototoxic effect of bumetanide is five times that of furosemide on a milligram-for-milligram basis. The ototoxic potential of bumetanide is one eighth that of furosemide when the doses are adjusted for diuretic potency difference between the two diuretics.

Comparative acute cochlear toxicity of intravenous bumetanide and furosemide in the purebred beagle

Comparisons were made of the effects of various doses of intravenous bumetanide and furosemide on the primary auditory afferent activity (N1) and cochlear microphonics (CM) of beagles. The dose-response relationships of the N1 depressions to bumetanide and furosemide are parallel; those of the CM depressions are also parallel but have a much shallower slope than those of the N1 depressions. With both drugs, N1 depression occurs at lower doses than does CM depression. The N1 depression produced by a particular dose of bumetanide or furosemide bore a linear relationship to the CM depression produced. This finding supports the postulate that the cochlear site and mechanism of ototoxic action of the loop diuretics are directed at an earlier step of the cochlear transduction process than N1. Using N1 depression as the gross electrophysiologic index of ototoxicity, the acute ototoxic potency of bumetanide in beagles is approximately 6.5 times that of furosemide, whereas its diuretic potency is 40 to 60 times that of furosemide. Therefore, when clinical dosages of the two drugs are considered, the relative acute ototoxic potency of bumetanide in the beagle is 0.11 to 0.16 that of furosemide. This range is identical to the relative ototoxic potency of 0.11 to 0.16 previously obtained in the cat. Serum concentrations of bumetanide and furosemide increased linearly with the doses of the two drugs, except for the highest dose given (100 mg/kg for both drugs). The serum concentrations at that dose of both drugs are less than the mathematically predicted values. Histologic (light-microscopic) examination of the cochleas did not reveal any significant pathology.

The Davis theory: a review, and implications of recent electrophysiological evidence

The Davis theory of mechano-electrical transduction asserts that the endocochlear potential and the hair cell resting potential summate to provide a driving force for current flow through the hair cell. However, while a variety of agents which depress the endocochlear potential simultaneously depress auditory nerve sensitivity and reduce the cochlear microphonic, recent reports suggest that the hair cells may be depolarised without such effects ensuing. The relevant literature is reviewed.

Effects of increased potassium in scala tympani on auditory nerve sensitivity

Raising the K+ concentration in scala tympani of the guinea-pig cochlea generally caused a substantial increase in the spontaneous firing rate of single auditory nerve fibres. This effect was not accompanied by any observed reduction in the threshold sensitivity of these fibres. These findings cast doubt on current theories of cochlear transduction.

Morphological alteration of the stria vascularis after administration of the diuretic bumetanide

Chinchillas were given either a single injection of the diuretic bumetanide (18 mg/kg body weight) or saline-sodium hydroxide and sacrificed at 10 min, 1 hr and 24 hr after the injection. Slight stria edema was present at 10 min, marked edema at 1 hr and no edema 24 h after bumetanide. The edema began in the first cochlear turn at 10 min and spread to the second turn by 1 hr. Along with edema, marginal cell bulging, potential capillary constriction and the formation of marginal cell membranous structures occurred after bumetanide treatment.

Elimination kinetics of furosemide in perilymph and serum of the chinchilla. Neuropharmacologic correlates

This study was done to determine the comparative elimination kinetics of furosemide from chinchilla perilymph and serum, and to correlate perilymph concentration with changes in endocochlear potential. The elimination kinetics of furosemide (FU) were determined in sera and perilymph obtained from chinchillas injected with 100 mg/kg i.v. of FU. Concentrations of FU exhibited a linear decay pattern in serum and perilymph over the initial 60 minutes. The rate of decline of furosemide levels in perilymph was about four times slower than the rate of fall in serum. Chronic treatment (25 mg/kg i.p. every 12 hours) did not appear to influence the level of drug at 60 minutes after a dose of FU (100 mg/kg IV). Chinchillas were also studied following doses of FU ranging from 25--200 mg/kg i.v. to see the effect on endocochlear potential (EP). A positive correlation was found between FU dosage, the maximum millivolt reduction of EP and the time to initiation of recovery of EP. The perilymph concentration of furosemide when the EP began to recover was 5 microgram/ml (1.5 x 10(-5) M). Knowledge of furosemide kinetics may ultimately be applied to prevent ototoxicity in patients.

Reduction of the endocochlear potential by the new "loop" diuretic, bumetanide

The effect of bumetanide upon the endocochlear potential (EP) was examined in 46 guinea pigs. The EP was reduced with dosages of 5 mg/kg or more. The maximum depression of the EP (reduction to -30 to -40 mV) was obtained at a dosage of 30 mg/kg. The recovery of the potential was incomplete at any dosage within three hours and the response pattern of the EP to bumetanide was similar to that to ethacrynic acid. The present experiments revealed that bumetanide, by weight, has a stronger ototoxic potency than the other "loop" diuretics--furosemide and ethacrynic acid. However, the diuretic effect of 1 mg bumetanide is equivalent to 40 to 60 mg furosemide or ethacrynic acid. Therefore, the relative ototoxic potency of bumetanide is much smaller suggesting that from a clinical standpoint bumetanide is much safer than the other "loop" diuretics.